1. Field of the Invention
The present invention relates to a charge read-out method and a solid-state imaging device, and specifically to a charge read-out method and a solid-state imaging device for reading out charges generated in a plurality of light receiving units arranged linearly.
2. Description of the Related Art
An apparatus using a line sensor for detecting light with a plurality of light receiving units linearly arranged has been heretofore known. For example, in some radiation image recording and reproducing systems, line sensors for detecting radiation images are incorporated.
The radiation image recording and reproducing system using the line sensor utilizes a storage phosphor (photostimulable phosphor), which stores a part of radiation energy when irradiated with radioactive rays (X-rays, alpha-rays, beta-rays, gamma-rays, electron beams, ultraviolet rays or the like), and then provides stimulated emission in accordance with the stored energy by irradiation of excitation light such as visible light. By utilizing the storage phosphor, the radiation image recording and reproducing system temporarily records radiation image information of a subject such as a human body on a storage phosphor sheet composed of a sheet-shaped storage phosphor layer, then scans the storage phosphor sheet with excitation light to generate stimulated emission light, and photoelectrically detects stimulated emission light with the line sensor to obtain image signals. Consequently, based on the obtained image signals, the system displays a radiation image of the subject as a visible image on a recording medium such as a photosensitive material or a display apparatus such as a CRT.
The line sensor receives the stimulated emission light generated from the storage phosphor sheet with a plurality of light receiving units arranged linearly, and moves charges generated and stored in the light receiving units to a charge transfer path disposed along a row of the light receiving units. Then, the line sensor transfers and outputs the moved charges through the charge transfer path for reading out the charges. Specifically, the charges stored in each of the light receiving units are moved from only one side of the row of the light receiving units to the charge transfer path.
A read-out time of charges stored in each of the light receiving units is determined on the sum of a time for moving all the charges generated and stored in each of the light receiving units to the charge transfer path and a time for transferring and outputting the charges moved to the charge transfer path along the charge transfer path.
The time for moving all the charges stored in the light receiving units to the charge transfer path is the time required for all the charges in each of the light receiving units to be moved at random and captured in the charge transfer path having a lower potential than that of the light receiving unit. The movement of the charges moving at random is mainly controlled by a repulsive motion of the charges (repulsive motion between charges of the same polarity) in the case of a large amount of charges, and is controlled by thermal diffusion in the case of a small amount of charges.
Since the stimulated emission light generated from the storage phosphor sheet is very weak, it is desired that a larger quantity of the stimulated emission light be received from the generated light. In order to receive a larger quantity of the stimulated emission light, taking into consideration diffusion of the stimulated emission light generated from the storage phosphor sheet, it is conceived that each of the light receiving units is increased in dimension in a direction (hereinafter, referred to as a cross-line direction) substantially orthogonal to a direction of a line in which the light receiving units are arranged linearly, thus increasing a light receiving area of each of the light receiving units. Accordingly, the quantity of the stimulated emission light received is increased.
However, a larger length of the light receiving unit in the cross-line direction requires a longer time for all the charges stored in the light receiving unit to be moved to the charge transfer path in accordance with the length, thus increasing the read-out time of the charges.
In other words, if the light receiving unit is increased in dimension in the cross-line direction (if the depth is lengthened without changing the exit size) while not changing in dimension of an exit of the charges through which the charges stored in the light receiving unit are moved to the charge transfer path (dimension with which the light receiving unit is electrically connected to the charge transfer path so as to move the charges thereto), the possibility that the charges moving at random in the light receiving unit are captured in the charge transfer path through the exit of the light receiving unit(the possibility per a unit of time), is lowered. Accordingly, the time for moving all the charges stored in the light receiving unit to the charge transfer path is increased.
As a result, the read-out time of the charges stored in the light receiving unit is increased, causing a problem that the charges stored in the line sensor by receiving the stimulated emission light, which is generated from the storage phosphor sheet, cannot be read out within a predetermined period of time.